- Bats Right, Throws Right
- Height 5' 9", Weight 164 lb.
He amazingly debuted in Belgium's top league as a 14-year-old, hitting .364/.400/.424 in 13 games for the Borgerhout Squirrels, who won it all.  He was one of the youngest players at the MLB Europe Elite Camp in 2015.  He was a regular for the Squirrels that year, batting .225/.325/.282 and fielding .947.  During 2016, the young infielder hit .284/~.432/.341 and fielded .912; he scored 26 runs in 28 games. He also pitched 3 times (1-1, 2.08). 
Janssen was the youngest player in the 2016 European Championship, over a month younger than Sweden's Max Hill, the next-youngest. He pitched twice. He relieved Robbe De Jongh in the 3rd with two on, no out and a 3-0 deficit against Italy. He retired Erick Epifano on a run-scoring grounder, allowed another run-scoring grounder to Marco Sabbatani, walked Stefano De Simoni, threw a wild pitch and retired Sebastiano Poma. He gave up one run in the 4th then allowed three of four runners to reach in the 5th before Matthew Joossens relieved and would let the two inherited runners in. In his other game, against Germany, he replaced Yannick Gontier with two outs in the 3rd and a 10-1 deficit. He allowed a Max Boldt single but got Christopher Howard to fly out then gave up one run in the 4th. In the 5th, he had a rough time, with a two-run homer to Howard and letting two more men on while retiring only one before De Jongh came in. His 15.75 ERA for the event was not Belgium's highest - Gontier, Gregg Mareels and Joossens were all higher. 
The right-hander hit .259/.329/.284 for the Squirrels in 2017, fielding .948 and posting a 1-1, 6.38 record. The team won the pennant.  He was 1-0 with a 1.69 ERA in the 2017 Federations Cup, beating the Leksand Lumberjacks, though he was 1 for 13 at the plate. The Squirrels won that event as well.  He was 2-0 with a 1.69 ERA in the European Championship U23 that year, beating Ukraine and Germany. He tied for the most wins in the event.  He also was part of the World Select Team for the Arizona Fall Classic showcase in the US, the lone Belgian chosen. 
Back in Belgium, he continued to be more of a position player even though he was only pitching for Belgium in international events. He hit .356/.414/.577 with 32 runs in 24 games and had a 3.86 ERA in his two starts, with no decisions. He fielded .902. He was 9th in the league in average, 3rd in slugging, tied for 6th in runs, tied for the most doubles (16) and tied for 9th in total bases (60).  He hit .222/.333/.222 in the 2018 CEB Cup but scored 7 runs in 5 games for the victorious Squirrels, including 3 against the Therwil Flyers. He tied Illia Velikki and Alexander Perdomo for 6th in the Cup in runs. 
He also played for Belgium in the 2018 Super6, pitching against the Czech national team in a 15-10 loss. He relieved Ben Van Nuffel in the 3rd and allowed 4 runs on 3 hits and 5 walks in 4 1/3 IP, fanning 7, before Gontier replaced him. He worked the most of any Belgian hurler that day and was one of their best, with only the 1/3 of an inning by Lino Ballardini being better.